Left hand shooter. 29& 1/4 draw. 66 inch longbow. 18 strand Astro flight string. 77lbs @ draw.  Bow will shoot 175 spine 300 spine and 400 spine. The bow will not shoot 250 spine or 340 spine. All with 315 points and all weighting around 830 to 950 grains. This does not make any sense to me.

Me either.

There must be other variables at play.  Hard to imagine 400 spine arrows working out of that setup and not being very week.

You didn't mention shaft length or diameter. But I agree, a 400 doesn't make any sense. Are we talking bare shafts, or feathered arrows, and are they all fletched the same?

"Longbow" covers a lot of territory. Is it a R/D type, a self bow, english longbow, or a Hill type? Does it have a sight window, or a shootaround grip.

I am not sure what you mean by those arrows shooting fine...I cannot imagine a situation where a bow would shoot 300 and 400 spine arrows and yet not shoot 340 spine just as well...so what information are you getting from someone watching you shoot those arrows over your shoulder?

DDave

as i just posted on another arrow spine thread ...

"carbon shafts have a far higher ratio of static to dynamic spine stiffness. this ratio is apparent as all carbon shafts list a very wide range of static spine, typically 15 to 20 pounds - you don't see that with wood, alum, alum hybrid or glass shafting.

that fact alone - the large STATIC spine range - can easily become even wider when DYNAMIC spine is considered. hence, it's very plausible that a 29" 500 static spine carbon arrow with 350 grains up front can fly very well out of a 55# holding weight bow ... or a 45# holding weight bow.

how you shoot a bow, how that bow is set up, all the parameters of the bowstring, and how your arrow is built, will all play MAJOR roles in how any arrow shaft will react upon your release of the bowstring, and how it will subsequently fly."